Fine-coarse drive mechanism

ABSTRACT

A pair of stepping motors form the basis of a coarse-fine drive mechanism. A coarse stepping motor imparts rotational motion through a drive gear to a driven shaft, and a fine stepping motor imparts linear or axial motion to the same shaft by means of a cam. The driven shaft is part of a system which integrates the afore-mentioned rotational and axial motions in order to rapidly and accurately move and position a rack through a wide range of drive increments which is afforded by the presence of the coarse and fine drives.

United States Patent Quilty [54] FINE-COARSE DRIVE MECHANISM JohnQuilty, c/o Photon, Inc,, 355 Middlesex Ave., Wilmington, Mass. 01887[22] Filed: Dec. 14,1970

[2]] Appl.No.: 97,799

[72] Inventor:

[30] Foreign Application Priority Data Dec 15, 1969 Great Britain..6l,089/69 [52] U.S. Cl... ..74/393, 74/398 [51] Int. Cl F16h 35/02,Fl6h 35/06 [58] Field of Search ..74/398, 396, 393

[56] References Cited UNITED STATES PATENTS 1,813,997 7/1931 Himes..74/398 [4 1 May 16, 1972 3,473,410 10/1969 Kraft ..74/396 X PrimaryExaminerLeonard H. Gen'n Attorney-William D. OReilly 57 ABSTRACT A pairof stepping motors form the basis of a coarse-fine drive mechanism. Acoarse stepping motor imparts rotational motion through a drive gear toa driven shaft, and a fine stepping motor imparts linear or axial motionto the same shaft by means of a cam. The driven shaft is part of asystem which integrates the afore-mentioned rotational and axial motionsin order to rapidly and accurately move and position a rack through awide range of drive increments which is afforded by the presence of thecoarse and fine drives.

5 Claims, 3 Drawing Figures PmNTEnnmamz 3.662.614

SHEET 1 [IF 2 INVENTOR JOHN QUILTY ATTORNEY FINE-COARSE DRIVE MECHANISMBACKGROUND AND BRIEF DESCRIPTION OF THE INVENTION This inventionpertains to a novel means for combining coarse and fine drive motions toobtain rapid drive speed while retaining accurate fine positioning. Theinvention may be employed to drive any equipment which must be movedrapidly, and then stopped with instantaneous accuracy, in anincremental, continuous manner.

In the past numerous arrangements have been used to move and position acarriage or other mass as quickly and accurately as possible. Typicallythese mechanisms include multiple solenoids, clutches and'brakes,escapements of various kinds, and the use of standard differentialdrives used in conjunction with servo or stepping motors. All the abovewere tried in an effort to find an accurate high speed positioningsystem but each was found to be unsuitable for one or more of thefollowing reasons: insufficient drive speed, poor positional accuracy,large backlash due to numerous gear meshes in the drive train, excessivecarriage bounce or overshoot causing relatively long settling time atthe desired position, or interaction between the coarse and fineportions of the drive system which caused excessive wear or inaccuracy.In order to achieve maximum speed and minimum settling time betweengiven positions, it is desirable, but not necessary, to make both coarseand fine positional corrections simultaneously.

It is therefore the object of this invention to provide a high speedcoarse-fine drive system which eliminates undesirable interaction of thecoarse and fine drive motions, and reduces the bounce and settling timeof the driven mass, while retaining precise positional accuracy. Theforegoing objective is attained in the present invention by utilizing afine drive stage which minimizes the interaction and inertia loadingbetween it and the coarse revolution stage, while providing thecapability of large drive increment differences between the coarse andfine drives.

The characteristic features and purposes of the present invention willbe obvious from the accompanying drawings and the following descriptionof the invention.

In describing the details of the invention, reference will be made tothe accompanying drawings in which:

FIG. 1 is a sectional view taken through the drive system showing theinventive drive arrangements;

FIG. 2 is a partial sectional view of the fine drive portion of thepositioning system; and

FIG. 3 is a modification which employs the same inventive principles.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIGS. 1 and 2, thecoarse drive section consists of a motor with a helical gear 12 on itsshaft 13 which drives a second helical gear 14 that is pinned to theoutput shaft 16. Attached to one end of this output shaft is the outputgear (or pinion) 18, which in turn drives rack 19, which may be acarriage or the like. When the fine drive section is not operating, itis the above coarse drive system that provides the rapid largeincremental drive motion to the output gear 18. Gear 14, shaft 16 andgear 18 may be hereinafter referred to as the first driven system, whilerack 19 may be hereinafter referred to as the second driven system. Thefirst driven system is adapted to integrate the rotational and axialmotion imparted to it by the coarse and fine drives, respectively, suchthat both coarse and fine incremental movement is possible in order torapidly and accurately position the second driven system.

The fine drive section can be best understood by referring to FIG. 2.When the fine drive motor 20 turns the cam 22 attached to its shaft 24,cam 22 moves shaft 16 containing helical gear 14 and output gear 18 inand out along its axis. The axial motion of helical gear 14 on outputshaft 16 causes gear 14 to rotate due to its angular mesh with helicalgear 12 on the coarse motor shaft 13, which is assumed stationary forexplanation purposes. If a 45 helical gear set is used, the axialdisplacement of output shaft 16 causes the output shaft to rotatethrough an angle (0 in radians) equal to the axial displacement dividedby the pitch radius of the helical gear. This fine movement of outputshaft 16 turns the output pinion or gear 18 providing accurate finalpositioning of a driven device 19 such as a carriage.

Cam 22 may be of symmetrical design which turns on either side of a 0position, or it can be a undirectional type which returns to 0 after Irevolution. In either case its total incremental range of movement mustbe able to rotate the output gear through the equivalent of the coarsedrive step angle (in other words, is equal to a single incrementalmovement of the coarse drive) to permit continuous positioningcapability.

The fine cam follower 26 as shown is prevented from rotating by the useof cam follower box 28, but guide pins, rails, slots, or many othermethods can be used to accomplish the same result. The positive actionof this box cam follower permits high speed position changes withoutbounce or overshoot. However, where lower speeds are involved, the boxedcam follower can be replaced by a spring 30 added to the system to loadthe output shaft 16 against the cam 22. The cam spring 30 shown in FIG.2 is used to take up any minor backlash that might develop from longterm wear of the cam or follower.

As may be readily seen the pair of helical gears shown in FIG. 1 may bereplaced with standard spur gears, in which case the output gear and itsdriven rack or gear would be changed to a helical pitch. If this weredone, the same drive action would result.

An alternate design which uses the same inventive drive principles isshown in FIG. 3. In this adaptation the line drive cam 40 provides axialmotion to the helical output gear 42 on the coarse motor shaft 44. Gear42 is keyed or splined at 43 to motor shaft 44, and is mounted on asleeve member 45 which is adapted to slidably fit over the splinedportion 43 of shaft 44, which allows gear 42 to move axially on theshaft without rotating. With the coarse motor shaft fixed, axial motionof the helical gear causes the helical rack (or helical driven gear) 46to move through the fine drive increment as previously described. In theembodiment of FIG. 3, gear 42, splined portion 43 and sleeve member 4may be referred to as the first driven system, and rack 46 as the seconddriven system.

Due to low inertia loading and minimum backlash of these drive systemsit is possible and desirable, although not necessary, to move both thecoarse and fine sections at the same time, thereby obtaining maximumspeed without undesirable bounce or overshoot. Previously, when standarddifferentials were used to mix coarse and fine motions, the cumulativebacklash of the many gear meshes required for large drive ratios as wellas interaction of both drives when operated together caused excessivebounce. Also, the combined inertia of the drive train limited themaximum speed attainable.

Although only two specific applications of the invention are shown inthis disclosure, it should be understood that the particular embodimentsdisclosed have been selected to facilitate explanation of the inventionrather than to limit the number of embodiments which it may assume.Further it should be understood that various modifications andadaptations may be applied to the specific embodiments described withoutdeparting from the spirit or scope of the present invention.

What is claimed is:

l. A coarse-fine drive mechanism comprising:

a coarse drive adapted to incrementally impart motion to a first drivensystem,

a fine drive also adapted to incrementally impart motion to said firstdriven system,

said coarse drive connected to said first driven system such that saiddrive imparts only rotational motion to said first driven system,

said fine drive connected to said first driven system in such a mannerthat said drive imparts only axial or linear motion to said first drivensystem,

said first driven system adapted to integrate said rotational and axialmotion imparted thereto whereby said first driven system is capable ofboth coarse and fine incremental movement in order to rapidly andaccurately drive and position a second driven system.

2. The drive mechanism of claim 1 in which:

said coarse drive comprises a stepping motor having an output shaft witha drive gear mounted thereon,

said first driven system includes a shaft having mounted thereon a gearpositioned so as to mesh with, and be driven by, said drive gear, and

said fine drive comprises a stepping motor having an output shaft with acam mounted thereon, said cam being positioned in operative associationwith an end of said driven shaft such that when the cam is rotated bysaid fine drive output shaft, said driven shaft is moved along its axis.

3. The drive mechanism of claim 1 in which:

said coarse drive comprises a stepping motor having an output shaftmounted thereon,

said first driven system includes a driven gear which is integrallykeyed or splined to said coarse drive output shaft such that actuationof said shaft rotates said driven gear, said driven gear being mountedon a sleeve member which is adapted to slidably fit over said splinedportion of the first driven system, and

said fine drive comprises a stepping motor having an output shaft with acam mounted thereon, said cam being positioned adjacent to an end ofsaid sleeve member such that when the cam is rotated by said fine driveoutput shaft, said driven gear is moved along its axis.

4. The drive mechanism of claim 1 wherein said coarse drive and saidfine drive are adapted to drive said first driven system simultaneously.

5. The drive system of claim 1 in which the total incremental range ofmovement of said fine drive is equal to a single incremental movement ofsaid coarse drive.

1. A coarse-fine drive mechanism comprising: a coarse drive adapted toincrementally impart motion to a first driven system, a fine drive alsoadapted to incrementally impart motion to said first driven system, saidcoarse drive connected to said first driven system such that said driveimparts only rotational motion to said first driven system, said finedrive connected to said first driven system in such a manner that saiddrive imparts only axial or linear motion to said first driven system,said first driven system adapted to integrate said rotational and axialmotion imparted thereto whereby said first driven system is capable ofboth coarse and fine incremental movement in order to rapidly andaccurately drive and position a second driven system.
 2. The drivemechanism of claim 1 in which: said coarse drive comprises a steppingmotor having an output shaft with a drive gear mounted thereon, saidfirst driven system includes a shaft having mounted thereon a gearpositioned so as to mesh with, and be driven by, said drive gear, andsaid fine drive comprises a stepping motor having an output shaft with acam mounted thereon, said cam being positioned in operative associationwith an end of said driven shaft such that when the cam is rotated bysaid fine drive output shaft, said driven shaft is moved along its axis.3. The drive mechanism of claim 1 in which: said coarse drive comprisesa stepping motor having an output shaft mounted thereon, said firstdriven system includes a driven gear which is integrally keyed orsplined to said coarse drive output shaft such that actuation of saidshaft rotates said driven gear, said driven gear being mounted on asleeve member which is adapted to slidably fit over said splined portionof the first driven system, and said fine drive comprises a steppingmotor having an output shaft with a cam mounted thereon, said cam beingpositioned adjacent to an end of said sleeve member such that when thecam is rotated by said fine drive output shaft, said driven gear ismoved along its axis.
 4. The drive mechanism of claim 1 wherein saidcoarse drive and said fine drive are adapted to drive said first drivensystem simultaneously.
 5. The drive system of claim 1 in which the totalincremental range of movement of said fine drive is equal to a singleincremental movement of said coarse drive.